The invention relates to data acquisition systems making it possible to convert a line made by a stylus on a surface into electrical signals constituting a point by point plot of said line. It more particularly relates to systems using elastic surface waves propagating on the surface of a piezoelectric plate. The end of the inscribing or marking stylus detects the electrical field created by the elastic waves in the immediate vicinity of the piezoelectric plate surface. The time taken by the elastic surface waves to reach the stylus is measured in two propagation directions. This chronometric measurement makes it possible to evaluate the distances separating the stylus tip from two electromechanical transducers alternately emitting pulses, whose wave fronts scan the complete plate. The acquired data are in the form of a succession of pairs of precise numerical values, which together form a point of point plot of the movement imparted to the stylus. The processing of said numerical values makes it possible to characterize the inscription and pattern for identification purposes, whilst a digital transmission system can be used for remotely reproducing a random graphic. Such a system forms the subject matter of French Patent Application No. 76 20 765, filed on July 7, 1976 by THOMSON-CSF and entitled "Locating System using Elastic Surface Waves".
In conjunction with a visual display console the operator responsible for making a line on the piezoelectric plate can check what he is writing with the stylus. However, this inscription control procedure is disadvantageous, because the line which is observed does not result directly from the action of the stylus on the plate. The electronic means used for correctly displaying the movements of the inscription are sufficiently complex to limit the development of data acquisition systems using elastic surface waves. The control of the making of a line flush with the surface on which the stylus moves is very natural and simple is the stylus tip leaves an easily erasable visible material mark.
The known inscription supports are well suited to such a control, because they have a texture permitting the attachment of an ink or a solid friable material. However, it is not possible to consider covering a support propagating elastic surface waves with a sheet of paper, because such a juxtapositioning would have the effect of disturbing the punctiform electrical propagation and detection modes of said waves.